1) Field of the Invention
The invention relates to drive systems for a motor vehicle, with an internal combustion engine and at least one electric machine, each of which can be used independently as a drive motor for the vehicle. The invention furthermore relates to methods for operating such a drive system. Drive systems of this kind, in which the internal combustion engine and the electric machine can each independently take over the driving of the vehicle are also known as xe2x80x9cparallel hybrid drivesxe2x80x9d.
2) Discussion of Related Art
One important aim in developing motor-vehicle drives is to reduce emissions of pollutants and noise caused by internal combustion engines. A critical area in this context is the drive-away phase of the vehicles: here, high specific fuel consumption and relatively high pollutant emissions occur. The latter is troublesome particularly in towns owing to the higher frequency of drive-away events in these areas. In addition, there is increased noise pollution due to the acceleration of the engines before clutch engagement.
In this context, parallel hybrid drives offer the possibility of accelerating the vehicle exclusively with the aid of an electric drive in the drive-away phase and only bringing in the internal combustion engine when the operating point is in a more favourable range as regards efficiency and pollutant emissions. One such solution is known, for example, from DE 33 35 923 A1. Here, after driving away electrically, the internal combustion engine is connected to the driven wheels via the electric machine by means of a clutch. The engine is started by the engagement of the clutch. A corresponding solution is known from EP 0 743 216 A2. There, the internal combustion engine is not coupled to the drive train mechanically but with the aid of an electromagnetic clutch. Coupling the stationary internal combustion engine to the rotating drive train has the disadvantage that a relatively large torque has to be supplied suddenly. This can lead to a jerky drop in the driving torque. To mitigate this unwanted effect, the internal combustion engine is constructed with a disengageable flywheel in the first-mentioned DE 33 35 923 A1. It is thereby possible to reduce its moment of inertia, allowing it to be xe2x80x9cbrought upxe2x80x9d more easily upon clutch engagement. In EP 0 743 216 A2, mentioned second, the jerky drop in torque is counteracted with a corresponding increase in the electric driving torque.
It is also known, with hybrid drives, to perform synchronisation during gear changes.
DE 195 30 231 A1, for instance, has disclosed a hybrid drive in which the electric motor is decoupled during gear changes and then at least approximately brought into synchronism with the drive shaft for re-engagement. There, the decoupling and coupling of the electric motor is used for simplified control of the synchronisation of the change gear. However, this is not related to the drive-away phase of the vehicle.
DE 195 39 571 A1 has furthermore disclosed a hybrid drive in which the internal combustion engine and the electric motor are coupled by means of an electrodynamic converter. The electric motor is used to bridge the gap between gears by synchronisation during gear changes. This is not related to the drive-away phase of the vehicle either.
According to a first aspect, a drive system for a motor vehicle is having an internal combustion engine and at least one electric machine, each of which can be used independently as a drive motor for the vehicle. The drive system is constructed in such a way that the drive-away phase of the vehicle proceeds as follows:
i) the vehicle is initially accelerated by the electric machine alone,
ii) during this process, the internal combustion engine is started,
iii) the internal combustion engine then takes over the driving of the vehicle,
jerky coupling of the internal combustion engine in the course of steps i) to iii) being avoided by entraining the internal combustion engine while the electric machine is accelerating the vehicle.
According to a second aspect, a drive system for a motor vehicle is having an internal combustion engine and at least one electric machine, each of which can be used independently as a drive motor for the vehicle. The drive system is constructed in such a way that the drive-away phase of the vehicle proceeds as follows:
i) the vehicle is initially accelerated by the electric machine alone,
ii) during this process, the internal combustion engine is started,
iii) the internal combustion engine then takes over the driving of the vehicle,
jerky coupling of the internal combustion engine in the course of steps i) to iii) being avoided by the internal combustion engine being accelerated for the purpose of starting while decoupled from the drive and being coupled to the drive at the synchronous speed.
According to another aspect, a method for operating a drive system for a motor vehicle, which is fitted with an internal combustion engine and at least one electric machine, each of which can be used independently as a drive motor for the vehicle, has the following steps:
i) the vehicle is initially accelerated by the electric machine alone,
ii) during this process, the internal combustion engine (1) is started,
iii) the internal combustion engine then takes over the driving of the vehicle,
jerky coupling of the internal combustion engine in the course of steps i) to iii) being avoided by entraining the internal combustion engine while the electric machine is accelerating the vehicle.
According to still another aspect, a method for operating a drive system for a motor vehicle, which is fitted with an internal combustion engine and at least one electric machine, each of which can be used independently as a drive motor for the vehicle, has the following steps:
i) the vehicle is initially accelerated by the electric machine alone,
ii) during this process, the internal combustion engine (1) is started,
iii) the internal combustion engine then takes over the driving of the vehicle,
jerky coupling of the internal combustion engine in the course of steps i) to iii) being avoided by the internal combustion engine being accelerated for the purpose of starting while decoupled from the drive and being coupled to the drive at the synchronous speed.
Other features which are inherent in the disclosed drive systems and methods will become apparent to those skilled in the art from the following detailed description and its accompanying drawings.